It is well known and also described in the literature that in potentiometric measurement of a whole blood sample and the corresponding plasma sample, deviating measuring results are often recorded between the two samples, and that this deviation is dependent on the erythrocyte content of the whole blood sample. This effect is usually called the suspension effect, cf. e.g. O. Siggaard Andersen, The Acid-Base Status of Blood, Munksgaard, Copenhagen, 1974, p. 156.
The deviating measuring results, which e.g. occur for a potentiometric measuring chain comprising sample, an ion-sensitive indicator electrode, a reference electrode and saturated KCl as salt bridge liquid between the sample and the reference electrode, are explained by the fact that the liquid junction potential between a whole blood sample and the salt bridge liquid is different from the liquid junction potential between a corresponding plasma sample and the salt bridge liquid.
The influence of the liquid junction potential on the measuring result obtained by potentiometric determination appears from the Nernst equation, which expresses the connection between the concentration of the ion I and the potential E of the electrode chain: ##EQU1## in which
E represents the potential of the electrode chain,
E.sub.o represents the standard potential of the electrode chain which is independent of the concentration of I,
E.sub.j represents the liquid junction potential,
RT/ZF is the Nernst factor, and
c.sub.l is the concentration of the ion I.
In the present specification and claims, the concentration of the ion I is to be understood as covering both the concentration and the activity of the ion I.
From the above equation it is clear that an electrode chain for which the Nernst equation applies can be used to determine the concentration of I, c.sub.l, provided that the electrode chain has first been calibrated with a solution with a known concentration of I and that E.sub.o +E.sub.j can be regarded as constant.
When measuring blood samples which are identical apart from varying erythrocyte content, it must be expected that E.sub.o is constant and that it is thus variations of E.sub.j which account for the suspension effect.
In connection with the reference to the suspension effect in the above-mentioned literature passage by O. Siggaard Andersen, various proposals for the elimination of the effect are given, viz. either to use a 0.15M KCl salt bridge or to insert a small bridge of plasma between blood and saturated KCl. Likewise, the possibility of adjusting for the suspension effect is mentioned.
These proposals each have various disadvantages:
by the normally occurring variations of the ion composition of blood samples, a 0.15M KCl salt bridge will give varying liquid junction potentials, as the liquid potential will then not be dominated by K.sup.+ - and Cl.sup.- -ions because of the low concentration of these ions;
insertion of a plasma bridge is complicated and not convenient for routine measurements of non-scientific character;
corrections can only be made according to a knowledge of the erythrocyte content of the sample, i.e. this requires equipment for the determination of an extra parameter.
Another possibility would be to establish a so-called flow junction in which the salt bridge liquid which contacts the sample is constantly renewed, but this solution is also inconvenient, especially because flow-conditioned potentials will occur.
In connection with the development of instruments for the determination of the potassium ion concentration in blood and plasma, it has been found inconvenient to use a salt bridge with a high content of potassium ions (2-3M) because of the risk that the sample, whose potassium content is about 4.times.10.sup.-3 M, is contaminated.
Therefore, alternative salt bridge liquids have been sought for, and in this connection, the usual criterion has been taken for the basis, viz. that the anion and the cation must have equivalent conductivity of very nearly the same magnitude, cf. e.g. Orion Research Inc., Newsletter/Specific Electrode Technology, Vol. 1, No. 4, September 1969, pages 21-23.